It's not really"dark side vs light side", rather its more like "rebels vs everyone else". Personally, my favorite faction is Jedi, but I've stopped using them in arena because they haven't gotten any new reworks/zetas in nearly a year and the only the only new characters they've got since then are either really hard to unlock (yoda) or also have the rebel tag (Ezra and Kanan).
Meanwhile, rebels have long had the strongest duos in the game, they got big buffs with rogue one, R2, and Luke's release, they got their own faction pass, and to top it off they're the faction most necessary for territory battles.
Other factions need love too